It only takes one trip to the range by a shooter unfamilar with Cordite loaded ammo to leave a bore in hideous shape, appearance wise anyway.
The previous owner of my No.1 was familar with small bore rifles, large bore pistols, shotguns, and BP revolvers. He'd fired the handloads I fixed up for him, and Winchester sporting ammo through the No.1 and knew how to clean modern powder fouling well enough, but when a gunshop owner gave him a free bandoleer of ammo it never occurred to him that this ammo would leave more fouling than the ammo he was used to or that the fouling it left would be more difficult to clean out of the bore.

Then after coming home from a shooting session he sprayed some solvent in the bore to let it soak overnight and when he put it in the gunsafe the shorter than normal guncase of his takedown rifle /shotgun combo fell over and jammed the locking bar so he couldn't get it open.
The No.1 sat uncleaned in the safe for months till members of another forum came up with ideas on how to get the safe open. He finally checked a safe of the same brand and took measurements, then drilled a 1/4 inch hole on the side of the safe and pushed the offending guncase aside using a shishkabob spit.
Something to remember if this ever happens to you guys.

Anyway he'd agreed to sell me this rifle as soon as he could get the safe open, I'd been keeping track of this particular No.1 Mk III for over 16 years planning to buy it from him the moment he found a rifle he liked better.

When I saw the bore I was shocked, it looked like roofing tar had been swabbed into the bore with a brush.
This was especially vexing since the main reason I wanted it was that it has the best and tiightest bore of any WW1 era enfield I've ever seen, near pristine when I cleaned it up for him when he first got it.
Luckily there appears to be no damage, whatever he sprayed down the bore seems to have prevented any pitting, even after so long a time uncleaned.
It took a lot of elbow grease and three different brands of solvent plus three new brushes to get all the crud out through.
I Checked it again last night and after I'd let Balistol work on it for a couple of weeks there was a slight bit of color to the patches which I figure to be the last vestiges of leading floating loose. Ten to one this final smear of leading will continue to come loose a wee bit at a time for some time to come.

A month back another old friend left his Mk III* here so I could take some photos. It looked clean as a pin, but I decided I might as well run a few patches through it since it had been in storage for several years.
Sweets patches showed no fouling at all, I then tried some balistol and the patches came out black as coal.
A patch got stuck and when I finally teased it out a chunk of lead dropped out that had been invisible to the naked eye when looking down the bore. He'd tried some cast bullet loads years ago and this chunk of lead had been ironed into the groove to be floated loose and rolled up by the balistol soaked patch.

Sweets gets out almost everything but apparently resin and lead can get past it.

Balistol seems to get out stuff other solvents can't touch. Great stuff.

As often as we find Enfield bores that are significantly oversized, as much as three or four thousandths, yet still shootable and accurate so long as bullets suited for the purpose are used, such as the Hornady .312 or others that slug out to fill the grooves. It seems to me that many Enfield bores that look to be a lost cause might be saved by lead lapping.

I've seen references to barrels marked as having been "draw lapped" but I'm not sure what is meant by that.
I've also seen references to some pre SMLE rifles having about half the bore lapped towards the muzzle end, to allow blowby and a resulting drop in pressures so they would hold up better to the higher pressure WW1 loadings. o lapping bores was not an uncommon process for the British armorers.

Any way if a slightly pitted bore that slugs out at .311-.312 or so were carefully lead lapped (not fire lapping but old style lead lapping) to no more than .314-.315 or so it should still be shootable with some degree of accuracy.
Mk VII bullets seem to slug out nicely and generally will shoot with decent accuracy in oversized bores.
I expect they were designed to do just that.
The German commission rifle if I'm not mistaken generally had a .321 bore yet the bullets were .318, with slugging meant to make up the difference. Bullets designed for the .32-40 Winchester and measuring .321 were often recommended for reloading the 8mm for the commision rifles.

I've miked 7.65 Mauser bullets that measured .314 at the base, but smaller further up, if these were available they'd be a good option for an oversized .303 bore.
And cast bullets of course can be sized to or bumped up to .314 or larger diameter if necessary.

Bumping up jacketed bullets is an old trick, many .22 Savage High Power owners found this necessary when .228 bullets were unobtainable.